Albinism is due to a mutation within one or several of the genes
coding for the pigment melanin. So, in the distant past an individual
and/or a group of people aquired the mutation and passed it on to
their offspring. Since this does not effect reproduction in any way the
mutant allele (alternate form of a gene) was able to remain within the
population.
Basically these individuals do not make enough of the pigment
melanin. Melanin is a photo-protective pigment. This is why one gets
a tan (skin becomes darker by long exposure to the sun), the body
produces more melanin in order to protect the body from UV light.
The darker your skin, the better you are able to ward off the damaging
effects of the sun's rays.
There are a variety of causes to the braod catagory of albinism as can
be noted by the references below.
http://www.albinism.org/infobulletins/what-is-albinism.htmlhttp://www.cbc.umn.edu/iac/facts.htmhttp://www.dircsa.org.au/pub/docs/albinism.htmhttp://www.albinism.org/infobulletins/hermansky-pudlak-syndrome.ht
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-Enjoy!